1. Field of the Invention
The present invention relates to a non-magnetic steel composition suitable for use in various steel and concrete structures, particularly magnetic floating high-speed railways, nuclear fusion facilities and marine structures and appliances where non-magnetic property is required.
The steel materials suitable for the above applications must have good corrosion resistance, and therefore the present invention also relates to a non-magnetic steel composition useful for preventing the decay of marine steel and concrete structures and similar structures which may be built on seashores in particular.
2. Description of the Related Art
In recent years, various methods for preventing the decay of steel and concrete structures which are built on the ocean and seashores have been proposed and indeed some of these have already been put into practice.
The principal cause for the decay of steel structure includes the corrosion by the seawater itself and corrosion by the sea salt particles. Meanwhile the principal cause for the decay of concrete structures has been found to be attributable to the fact that reinforcing steel bars or wires embedded in the concrete structure are corroded by salts contained in sea sand used when mixing the concrete or by sea salt particles which permeate into a concrete structure built on a seashore or in the seawater. The corrosion salts have an increased volume of about 2.2 times, and the concrete fails to withstand the expansion forces of the corroding steel bars or wires. The concrete thus cracks along the embedded reinforcing bars or wires. When the cracks grow to about 0.2 mm or larger, external corrosive media, such as oxygen, salts, and carbon dioxide in the air, penetrate these cracks to reach the interior of the concrete mass where the reinforcing bars or wires are embedded. This further promotes the corrosion of the bars or wires, or accelerates neutralization of the concrete, causing premature decay of the concrete structures.
Also, in recent years, trials have been made to prepare steel materials containing 15% or more manganese for the purpose of obtaining the non-magnetic property, but one critical problem confronting all of these Mn-containing steel materials is that the rust generation rate is remarkable, higher than ordinary carbon steels, hence a higher corrosion rate in the presence of a very small amount of salt.
Therefore, a main object of the present invention is to completely prevent the corrosion of structures built with non-magnetic steel materials and the concrete decay of concrete structures reinforced with non-magnetic steel wires, which may be built on the seashores. These problems of steel corrosion and concrete decay in the marine environments have been given keen attention in various fields of industries.
More imminent problems now to be solved are in connection with concrete structures more than 20 years old. In many fields, the free salt content around the reinforcing bars or wires embedded in the concrete structures may be as high as 1.0% in terms of NaCl in severe marine environments, and this causes serious corrosion of the reinforcing bars or wires, which in turn causes and promotes cracking of the concrete.
Therefore, it has been strongly desired to resist attack by a high concentration of free salt, thus almost completely eliminating the possible corrosion of steel structure and cracking of a concrete structure, which may be exposed to a very high concentration of salt.
The above objects of the present invention can be achieved by building steel structures and reinforcing concrete structures with a seawater-corrosion-resistant non-magnetic steel material containing not more than 1.0% carbon, not more than 0.25% silicon, not more than 2.0% manganese, more than 20.0 up to 37.3% aluminum, 1.0 to 5.5% chromium, not more than 0.015% phosphous, and not more than 0.005% sulfur, optionally with one or more of Ti, V, Nb, W, Mo, and B, in an amount ranging from 0.01 to 0.5%, in total for the elements other than B, and in an amount ranging from 0.0001 to 0.005% for B, and one or more of Cu and Ni in an amount ranging from 0.1 to 5.5%, in total, with the balance being iron and unavoidable impurities.
The most important features of the present invention reside in that a relatively large amount of Al is contained in the steel so as to lower the Si and S contents in the steel and also to obtain a stabilized non-magnetic property.
The advantage obtained by the limitation of Si and S contents in steel and the relatively large contents of Al are described as follows. The lowered Si content in the steel will suppress the formation and growth of rust and the content of MnS which creates the nuclei for rust formation is markedly lowered along with the lowering of S content in the steel so that the deterioration of corrosion resistance can be minimized, and the increased Al content in the steel will strengthen the passivated film formed on the surface of the high-manganese steel so that the passivated film, even exposed to a high concentration of salt, is not destroyed, thus preventing the rust formation.
An explanation will be made as to the reasons for limiting the contents of the individual elements as defined in the present invention.
Carbon is limited to an amount of not more than 1.0% for the reason that more than 1.0% carbon will cause embrittlement of the steel. A lower carbon content is more desirable because carbon has a large tendency, when heated during heat treatments, to form magnetic complex carbides consisting of Fe, Al and C. A preferable carbon range is from 0.001 to 0.1%.
The reason for limiting the Si content to an amount not more than 0.25% is that Si is necessary to assure the required strength of the steel and to control the non-metallic inclusions, but a lower Si content will markedly suppress the rust formation. For these conflicting purposes, the Si content is limited to an amount not more than 0.25%. A preferable Si content is not more than 0.05%.
The Mn content is limited to an amount not more than 2.0% because Mn contents more than 2.0% will cause difficulties in hot rolling. From the point of rust prevention, Mn contents not more than 1.0% are preferable.
The P content is limited to an amount not more than 0.015% for the reason that P contents more than 0.015% produce no effect to suppress the rust formation in an alkaline environments such as concrete, but rather tend to promote the rust formation.
Aluminum is the most important metal element in the steel composition according to the present invention. And the reason for limiting the Al content to an amount ranging from more than 20.0 to 37.3% is that with Al contents of 20% or less the de-magnetization of the steel is not sufficient, but with Al contents more than 37.3%, there is a great tendency to produce intermetallic compounds between Al and Fe, which cause embrittlement of the steel, thus prohibiting the hot rolling. A preferable Al content ranges from 20.5 to 28.0%.
Chromium is limited to an amount ranging from 1.0 to 5.5%, for the reason that a chromium addition will improve the corrosion resistance in such a severe corrosion environment as exposure to alternate wetting by sea water and drying, and hot workability of steel when Al content is 20.0% or more, but a chromium content of more than 5.5% is not economical.
The S content is limited to an amount not more than 0.005% for the purpose of reducing the content of MnS which is the cause for the formation of rust. Incidentally, Ca and rare earth metal elements used as desulfurization agent to lower the S content may convert MnS into (Mn, Ca)S and so on thereby additional corrosion resistance improvement can be expected.
The above procedure for lowering the sulfur content is a common practice in the art and it is very often that the steel contains a small amount of Ca and rare earth metal elements as Ce, but the presence of these elements is permissible because they will not produce adverse effects on the corrosion resistance of the steel.
According to the present invention, Ti, V, Nb, W, Mo, and B may be added when desired to improve the strength and toughness of the steel as is conventional. One or more of these elements are added in an amount ranging from 0.01 to 0.5% for a single element or in combination for the elements other than B, and in an amount ranging from 0.0001 to 0.005% for B. The addition of these elements for the above purposes is conventionally known.
Further, when required, one or more of Cu, and Ni may be added in an amount ranging from 0.1 to 5.5%, in total.
Still further, for applications such as screwed concrete reinforcing wires where free cutting property is required, 0.01 to 0.5% Pb may be added.
A steel having the chemical composition mentioned hereinbefore may be prepared by melting in a converter or electric furnace, then the steel is subjected to ingot-making and breaking-down, or to continuous casting, then to rolling and heat treatments such as quenching, annealing, normalizing and patenting, if necessary, and finally drawing into bars or wires for final use. However, the final products may be supplied in the forms of pipes, H-sections, concrete reinforcing bars, wires, and sheets, and if necessary, may further applied with Zn coatings and organic coatings.